This invention relates to a device for detecting a relative position and a relative moving speed between two bodies which rotate and move relatively, and more particularly, to a position and speed detecting device of a rotating body which is most suitable for obtaining control signals for phase servo or speed servo in a rotating driving device, such as a rotating magnetic head in a VTR (Video Tape Recorder) or the like, speed control of a disk motor of a magnetic disk apparatus, or obtaining position signals, such as index signals on a disk or the like.
In general, in a rotating driving device, such as a rotating-type magnetic head device or the like, a phase detector and a speed detector (a speed control circuit) for detecting the rotation phase and rotation speed of a rotating body is disposed in order to rotate the rotating body, such as a magnetic head drum and the like, at a constant speed in a predetermined rotation phase, and phase servo and speed servo are performed for a driving motor utilizing each detected output. In a brushless motor used as the driving motor, it is necessary to sequentially excite its stator coil according to a predetermined drive sequence in accordance with a rotation angle position of rotor magnet. Hence, a position detection circuit for detecting the rotation angle position of the rotor magnet is disposed in the brushless motor. In the following description, a combination of an FG (Frequency Generator) coil pattern and a rotor magnet may be described as a "detection-signal generator".
The various configurations of such a conventional position and speed detecting device are disclosed in Japanese Laid-open Jikkaisho 187406/1986, 193318/1986, 193319/1986 and Tokkaisho 117460/1983.
According to them, one of conventional position and speed detecting devices uses a circular FG coil pattern of serially connected multiple coil sections of which a pitch of the coil section is partly modified to have a finer pitch in contrast to other part of the circular FG coil pattern, i.e. in such a coil pattern, each of the 2.sup.n th (n=0, 1, 2, 3 . . . ) coil sections of the multiple coil sections has a narrow pitch which is smaller than a half of the pitch of other coil sections. And a rotor magnet pattern of alternated magnetic polarity sections associated with the circular FG coil pattern is correspondingly subdivided to be smaller than a half of the pitch of other magnetic polarity sections at the 2.sup.n th positions correspondingly with the 2.sup.n th coil section.
It is possible to provide the FG coil pattern and rotor magnet formed in 1/2 pitch at only one location. Such a configuration can be realized as a combination of disks facing each other, or as a plane and a cylindrical surface.
A signal which is taken out from the coil pattern and which has a higher threshold value between two pulse circuits is taken out as a standard position signal, and a signal from another pulse circuit which has a lower threshold value is taken as a rotation-speed detecting signal, respectively.
In a conventional position and speed detecting device as described above, it is necessary to make the relationship between an equidistant pitch P.sub.1 and a pitch P.sub.2 a width of 2:1 at a predetermined position, with the distribution of magnetic flux varies in accordance with the amount of pitch, and therefore there is a disadvantage that the output level of an electromotive force e which is generated by a magnetic pitch for a gap d and which is determined by B.multidot.l.multidot.v=e (B: effective magnetic flux density which the FG coil receives, l: FG coil length interlinking the moving direction of the magnetic pole, v: relative moving speed) is small. Particularly, in a conventional device which has a detection-signal generating unit having a narrow pitch at 2.sup.n -th positions, the width of a narrow pitch is still smaller than 1/2 when compared with the equidistant pitch of the rotor magnet. Hence, there is a disadvantage that the output level of an electromotive force generated by a magnetized pitch for a gap d becomes still smaller.